1. Field of the Invention
The present invention relates to a sensor assembly. More specifically, this invention relates to a non-invasive, media isolated, and stress isolated pedestal mounted sensor die exhibiting improved sensitivity, and reduced measurement error caused by induced substrate stress, and enhanced media isolation, and method therefore.
2. Description of the Related Art
Sensors convert the absolute value or change in a physical quantity such as temperature, pressure, flow rates, pH, and the like into a useful input signal, typically electrical signals, for an information-gathering system to permit measurement and control of the environment in which that sensor is located. The particular physical parameter to be measured generally dictates the type of sensor to be used. For example, a pressure sensor is typified by a piezoresistive element disposed on or incorporated into a flexible diaphragm integrated into the sensor die. Stress caused from a pressure differential across the diaphragm causes the diaphragm to deform resulting in a measurable change in the resistance of the piezoresistive element, thereby permitting calculation of the pressure. Flow rate sensors may similarly employ a piezoresistive strain gauge for measuring the flowrate of a gas or liquid stream (i.e., a fluid stream). These sensors are typically inserted into the liquid or gas stream for which a measurement is desired.
Sensors are typically inserted directly into the media, or fluid, stream whereby contemporaneous measurements of pressure, temperature, flow rates, and the like may be made as the values for these parameters change. However, these invasive measurements of physical parameters may place special demands on a sensor, particularly where the temperature, corrosivity, or solvating capacity of the media are incompatible with the sensor assembly construction and/or components. Chemical incompatibility may limit, or completely deny, the use of a particular sensor where the media may result in destruction of the sensor. For example, a sensor used to measure the pressure or flow rate of a hot solvent may be short-lived in view of the various soluble components which may comprise the sensor assembly, particularly glues, epoxies, plastic components, and the like. Similarly, a corrosive media may result in oxidation and destruction of the organic and metal components of the sensor.
The temperature of the media is another environmental factor that may affect the performance of a sensor. The problem of temperature and temperature differentials and their resulting stresses generate a strain in the sensor assembly causing errors in the measurement, and reducing the sensitivity of the measurement. Generally, temperature compensation circuits are employed to improve the precision of the sensor measurement. However, those additional components comprising the temperature compensation circuit, as well as the conductive traces interconnecting these components, are susceptible to attack by both solvents, corrosive media, and high temperatures. Moreover, the stress associated with differences in thermal expansion coefficients between the various dissimilar materials from which a sensor assembly is constructed inevitably leads to strain across the piezoresistive element, resulting in errors such as offset bias, and response slope errors.
The means by which a sensor die is attached to the substrate also affects the performance of the sensor. Sensors presently available disfavor a "hard" attachment, i.e., a soldered junction, between the sensor die and the substrate. Hard attachments are considered mechanically unstable, and result in performance problems caused by so-called material creep. The resulting stresses are transmitted from the substrate to the sensor element and render the performance of the sensor unstable. Accordingly, sensors presently available are typified by a "soft" attachment whereby an adhesive, such as epoxy adhesives or silicon adhesives, is used to attach the sensor die to the substrate. These soft attachment assemblies, however, are unsuitable in corrosive environments and are unable to tolerate temperature extremes, becoming brittle at low temperatures. Even where a protective coating is applied over the adhesive, any pinholes in the coating will permit a corrosive fluid, such as an acid, or acidic gas, to attack the adhesive resulting in a sensor failure within a short period of time.
Various attempts have been made by those practicing in the art to address the problems associated with a hostile sensor environment. Boyer, in U.S. Pat. No. 5,257,747, issued Nov. 20, 1993 discloses a pressure transducer comprising a pressure sensor mounted to a buffer plate, which in turn is mounted to a ceramic plate. The ceramic plate is supported within a housing, and is separated from all parts of the housing except for a central surface on a support boss. A cover is attached to the housing. The housing and cap are constructed to provide offset positioning of the housing with respect to an external device to isolate the housing from sources of external stress. While this configuration may provide some stress isolation from forces occurring external to the housing, it does not provide for stress isolation of the sensor from the ceramic substrate. The sensor is bonded by substantially its entire surface area to the buffer plate which in turn is bonded by substantially its entire surface area to the ceramic substrate. Although Boyer teaches a flexible adhesive to permit relative movement of the ceramic substrate to the sensor die, any stress resulting from temperature differentials between various areas of the ceramic plate, or resulting from temperature differentials between the media being sampled and the ceramic plate are easily transmitted through the flexible bonds and to the sensor. Moreover, the sensor, buffer plate, and mounting cement are exposed to the fluid media, thus providing no isolation from the media. Since Boyer's flexible media suggests an organic adhesive, even were a protective coating applied to the exposed areas of the sensor, any leaks or pinholes in the coating renders the mounting adhesive susceptible to chemical oxidation and degradation from corrosive and solvating media.
U.S. Pat. No. 5,483,994, issued to Maurer on Jan. 6, 1996 discloses a pressure sensor comprising a dual chamber divided by a flexible membrane. Fluid is introduced into the first chamber, thereby deforming the flexible membrane. A plunger having an elongated stem is disposed in the second chamber and is responsive to the membrane movement. The distal end of the plunger stem is in contact with a pressure sensor. As the fluid pressure increases, the membrane presses against the plunger, whose stem presses against the sensor thereby providing an indication of the fluid pressure. While the fluid is isolated from the sensor by the membrane and plunger, critical tolerances must be adhered to ensure that the plunger and stem just touch the membrane and sensor at all times to provide sensitivity at low pressures. If the plunger and stem are too short (for example, contraction of the stem in cold weather), there is a latency, and a threshold pressure must be exerted before the stem contacts and presses against the sensor. Moreover, in view of the thermal expansion of the plunger and stem, high temperature environments may cause elongation of the plunger and result in a stress on the sensor stem creating an erroneous offset pressure signal. Finally, Maurer's device does not provide a means for isolating the sensor from external stresses. As shown, a conductive seal is disposed directly below the sensor die, and a die support is disposed directly above the sensor die, thus placing the sensor die in intimate physical conduct with the sensor body, or housing, resulting in significant exposure to stresses caused by thermal and external forces.
U.S. Pat. No. 5,067,491, issued to Taylor, II et al. on Nov. 26, 1991 is directed to a media isolated sensor probe mounted on a catheter adapted for invasive insertion into, and monitoring of, a patient's blood stream. The sensor element and catheter are coated with a thin layer of polymer to provide media isolation, thus isolating the sensor and catheter from bioreactive and other deleterious materials in the blood stream. Moreover, since Taylor's probe must be inserted in its entirety into the fluid (i.e., the bloodstream), the size of the probe must remain small, thus Taylor does not provide for stress isolation of the sensing element.
The devices of the prior art fall short of providing a sensor assembly having both stress isolation and media isolation. There is a need for a sensor assembly for non-invasive monitoring of physical parameters associated with a liquid or gas stream that provides both stress isolation and media isolation of the sensor assembly to permit both greater sensitivity and reliability.